Long-Term Evolution (LTE) uses orthogonal frequency-division multiplexing (OFDM) in the downlink and discrete Fourier transform (DFT) spread OFDM (DFTS-OFDM) in the uplink. DFTS-OFDM allows for flexible bandwidth assignment and orthogonal multiple access not only in the time domain, but also in the frequency domain. Thus, the LTE uplink scheme is also sometimes referred to as Single-Carrier FDMA (SC-FDMA). In the time domain, LTE downlink and uplink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. The resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) (i.e., there are two slots per subframe) in the time domain and 12 contiguous subcarriers in the frequency domain.
To support the transmission of downlink and uplink transport channels, certain uplink layer 1 and layer 2 (L1/L2) control signaling is used. The uplink L1/L2 control signaling includes: (a) hybrid-automatic repeat request (HARQ) acknowledgements (ACK/NACK) for received downlink data; (b) channel-status reports related to downlink channel conditions, which reports may be used by the base station in scheduling the transmission of data in the downlink; and (c) scheduling requests indicating that the mobile terminal (a.k.a., “user equipment (UE)”) needs uplink resources for uplink data transmissions. For example, after receiving downlink data in a subframe from a base station, the UE attempts to decode the data and reports to the base station whether the decoding was successful (ACK) or not (NACK). In case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data.
A UE should transmit uplink L1/L2 control signaling regardless of whether or not the UE has any uplink transport-channel (UL-SCH) data to transmit and, thus, regardless of whether or not the UE has been assigned any uplink resources for UL-SCH data transmission. Hence, two different methods are used for the transmission of the uplink L1/L2 control signaling, depending on whether or not the UE has been assigned an uplink resource for UL-SCH data transmission.
In case the UE does not have a valid scheduling grant—that is, no resources have been assigned for UL-SCH in the current subframe—a separate physical channel, the Physical Uplink Control Channel (PUCCH), is used for transmission of uplink L1/L2 control signaling. Otherwise, the uplink L1/L2 control signaling is multiplexed with the coded UL-SCH onto the Physical Uplink Shared Channel (PUSCH).
More specifically, if the UE has not been assigned an uplink resource for data transmission, the L1/L2 control information (e.g., channel-status reports, HARQ acknowledgments, and scheduling requests) is transmitted in uplink resources (resource blocks) specifically assigned for the uplink L1/L2 control information on the PUCCH. These resources are located at the edges of the total available cell bandwidth. Each such resource consists of 12 “subcarriers” (one resource block) within each of the two slots of an uplink subframe.
LTE release-8 (Rel-8) has recently been standardized. LTE Rel-8 supports bandwidths up to 20 MHz. The Third Generation Partnership Project (3GPP) has initiated work on LTE release-10 (Rel-10). One of the parts of LTE Rel-10 is to support bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular, for early LTE Rel-10 deployments, it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CCs, where the CCs have, or, at the least, have the possibility to have, the same structure as a Rel-8 carrier.
The CA PUCCH is based on DFTS-OFDM for a UE supporting more than 4 ACK/NACK bits. The multiple ACK/NACK bits (may also include scheduling request (SR) bits) are encoded to form 48 coded bits. The 48 coded bits are then scrambled with cell-specific (and possibly DFTS-OFDM symbol dependent) sequences. The first 24 bits are transmitted within the one slot and the other 24 bits are transmitted within a second slot. The 24 bits per slot are converted into 12 QPSK symbols, spread across five DFTS-OFDM symbols, DFT precoded and transmitted within one resource block (bandwidth) and five DFTS-OFDM symbols (time). The spreading sequence is UE specific and enables multiplexing of up to five users within the same resource block. A demodulation reference signal is also transmitted in each slot. The reference signal comprises a reference sequence. As used herein, a “reference sequence” may represent any information transmitted by a transmitting device to permit or otherwise facilitate the demodulation, by a receiving device, of data associated with the reference sequence (e.g., data transmitted with the reference sequence). For example, in particular embodiments, the reference sequence may represent a cyclic shifted CAZAC sequence (e.g., the computer optimized sequences in 3GPP TS 36.211). To improve orthogonality among reference signals even further, an orthogonal cover code of length two can be applied to the reference signals.
In the continuing evolution of the LTE system, additional transmit antennas are introduced to the UE to improve transmission performance. There is, hence, a need to design transmission methods and apparatuses that combine the benefits of DFT precoding and transmit diversity coding for PUCCH transmission (including CA PUCCH transmission).